Instrument approach and landing system for aircraft



Sept' 13 1960 N. A. BEGovlcH ETAL 2,952,845

INSTRUMENT APPROACH AND LANDING SYSTEM FOR AIRCRAFT Filed Sept. 16, 1955 7 Sheets-Sheet 1 Lnlalkvl.

7 Sheets-Sheet 2 Sept. 13, 1960 N. A. BEGovlcH ErAL INSTRUMENT APPROACH AND LANDING SYSTEM FOR AIRCRAFT Filed Sept. 16, 1955 sept- 13, 1960 N. A. BEGovlcH ET AL 2,952,845

INSTRUMENT APPROACH AND LANDING SYSTEM FOR AIRCRAFT 7 Sheets-Sheet 3 Filed Sept. 16, 1955 Sept. 13, 1960 N. A. lBEGovlczl-l ET AL 2,952,845

INSTRUMENT APPROACH AND LANDING SYSTEM FOR AIRCRAFT Filed Sept. 16, 1955 7 Sheets-Sheet 4 Irfan/iff.

Sept.-13, 1960 l N. A. BEGovlcH ETAL 2,952,845

INSTRUMENT APPROACH AND LANDING SYSTEM FOR AIRCRAFT Filed sept. 1e, 1955 7 Sheets-Sheet 5 5ans Sept- 13, 1960 N. A. BEGovlcH ET AL 2,952,845

INSTRUMENT APPROACH AND LANDING SYSTEM FOR AIRCRAFT '7 Sheets-Sheet 7 Filed Sept. 16, 1955 United States Patent lNSTRUlVIENT APPROACHAND LANDING SYSTEM FOR AIRCRAFT Nicholas A. Begovich and Herman Epstein, Los Angeles, and Richard H. Reed,'Gardena, Calif., assgnors to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed sept. 16, 195s, ser. No. 534,641

s claims. (c1. 34a- 5) This invention relates to systems for aiding the approach and landing of an aircraft on an airport runway and more particularly to apparatus capable of radiating electric energy capable of providing the aircraft with information for directing it along a predetermined descent path within the approach volume to the runway.

2,952,845 Patented Sept. 13, 1960 ICC ` different locations in the vicinity of the runway to produce a unique correspondence between frequency and angular coordinates'within the approach volume of the runway. In accordance with the present invention, a rst series of vertical beams is radiated from one locationand Y a second series of flat beams is radiated from a second location to provide information for the determination of The inadequacies of existing instrument landing systems (ILS) in regions close to the touchdown area have ,Y

been such as to confine their use to runway approach rather than'to the actual landing of aircraft. These shortcomings have diverted development veiort toward different techniques of providing landing position data to the aircraft. In one ofthese techniques, the sO-called ground control approach system (GCA), the aircraft is observed from the ground by means of radar, and directed by radio voice communications to the landing strip. Unfortunately, the errors in the data obtainable from a ground-based radar are again largest when the aircraft radar echo. Small changes in the aspect of the aircraft, 'Y

such as occur in mild cross winds orfgusts, cause appreciable uncertainty in a radar determinationof the .precise position of the aircraft, or of any particular point on the aircraft. 'Ihis is further aggravated in that the distance from the wheels of an aircraft to the mean posi'- tion of the center of gravity of its radar reflection varies `fromone aircraft configuration to another, so that the determination of the altitude of the aircrafts landing wheels above the runway is extremely diiiicult.

The -failure of the present so-called instrument landing systems results largely from their inability to define a smooth, accurate glide Vpath close to the runway touch down area. This difficulty is caused by the-distortion of the glide path pattern close to the touchdown area resulting from the method used to formthe pattern, and by pattern distortion produced by reflections-,from the ground and other obstacles. The obvious method of reducing the pattern distortion by radiating a narrower antenna beam is not available in the `conventional ILS for two reasons. `First, at the frequencies presentlyvused for ILS, suliicient reduction in the beamwidth ofthe radiated glide path pattern snot achievablewithout unreasonable antenna height. VMoreover, an increase-'of antenna height would result in the Fresnel zone of the radiation pattern extending tens of miles, thus again re-` leans.t More particularly, the rst problem iSSQ1V,0d`bY x the azimuth and elevation, respectively, of the aircraft. Thus, by the relatively simple measurement in the aircraft of Ithe frequency of the electromagnetic energy striking the aircraft, its angular position with respect to the runway can be uniquely and accurately determined. The multiplicity of narrow antenna beams results in an overall angular volume coverage equal to that of the present low frequency ILS and in addition eliminates the glide path distortions resulting from the use of broad antenna beams. f The introduction of frequency as aunique measure of angular position with respect tothe runway enables the use of extremely simple position measuring equip.- ment in the aircraft. That is, an aircraft may determine its angular coordinateswith respect to the runway by measuring the frequenciesl of the azimuth and elevation beams which areincident on the aircraft. In addition, a suitable glide pathV may be provided by the off-center intersection of the vertical azimuth v'beam with a -at conically shaped elevation beam that is tilted towards the takeoff direction of the runway. In this case, frequency discriminators adapted to generate null outputs for frequencies corresponding to the desired glidev path may be located in the aircraft to provide the pilot with information for directing the aircraft along theglide path. The null information thus provided is extremely precise as it is obtained by 'the beam'splitting of .very narrow beams. A

In 4addition to the above, the use of high operating frequencies for the different antenna beams makes possible the monitoring of the aircrafts position on the ground by utilizing the reflected radar echo from the aircraft. The ground based transmitters which generate the azimuth and elevation beams thus serve a dual function, i.e., they produce a unique frequency-versusangularpo sition space for measurement of position in the aircraft and they produce radar echoes from the aircraft so that ground-based receivers can monitor the position of the aircraft by radar techniques. Consequently, it is. seen that the instrument approach and landing system of the present invention provides data of sucien-t accuracy for the actual landing of the aircraft and, in addi-tion, provides ground control approach information for the monitoring of the aircrafts position from the ground.

The simplest method of generating the multiplicity of narrow antenna beams, each beam at a different frequency and at a different angular position in space, is to utilize' frequency scan radar techniques. These 'techniques consist of using an antenna whose radiated beam position is a function of the operating frequency ofthe radar. By changing the frequency of the radar in discrete increments, the antenna radiation pattern ca n made to step-scan in one plane. Inasmuch as the change in frequency may be performed electronically, extremely rapid scan rates may be obtained, thus providing practically continuous vposition -data in the aircraft and mo'nitoring data on the ground. The employment of frequency scan radar techniques results in -an appreciable improvementA in performance compared to conventional monochromaticmechanical scanning radars from-zthe "r standpointof `data rate, rejection of `ground cluttersig nals, and accuracy of antenna beam positioning. Since frequency scan beam positioning is far less dependent on mechanical tolerances, "a scanning vsystem lof 'high Aangular Aprecision is Aobtainable which requires 'a rniniin'um of adjustments and maintenance.

I t is therefore an object of the invention 'to provide apparatus capable of "radiating a multiplicity of na'row beams, each of a diiercnt'frequenc'y, from'at'leas't two different locations in the vicinity'of a-runv'vay rlthereby t0 produce a correspondence between Yfrequency and angular `coordinates within the approach volume `to therunway. "nother vobject of the 'device ofthe present invention is to lp'ovide an 4'aircraft"with accurate infomation 'to enable vitto approach and land on a runway along 'an improved localizer glide path.

`Still another object of the'device of `the `-present -invention 'cis `to provide accurate infomation 'to an aircraft 'to v"enable it'to approach -and land on a'ru'nway along 'a predetermined glide path'formed 'by thefintcrsection of atleast two :narrow beams, veach of ia'different freqnencyand :radiated from dilerent locations in the vicinityfof the runway.

A further object of the device of the 'present Iinvention is rto 'provide an aircraft with glide -path informatio'nfto enable it to approach and land on a -runway and, simultaneously, to provide ground personneltwith a radar presentation to enable" them to monitor'the `landin`g i Theriov'el features which are believedto'becharacteristie ofthe invention, both as toiit's organization vand method -of operation, together with-further'objects advantages thereof, will be better understood from 'the following description considered in connection lwith"th'e accompanying drawingsin which an lembtdiment'of lthe invention is illustrated'by way of example. It isftojbe expressly understood, however, that thedawings are Afor th'epurpose of illustration and description-only, and are 'notintended as a definition of the limits of the invention.

Fig. l shows ablo'ck diagram of lthe ground installation portion of the system of the present invention;

Figs. 2 and 3 are, respectively, 'a perspective andplan view showing schematically a typical airport installation ofthe device of the present'invention; V

Fig. 4 is 'a schematic diagram of a frequency 'shift antenna;

Figs. '5, 5a and 5b show the azimuth frequency :shift antenna illustrated schematically in Fig. 1; l

Figs. '6, and 6a show -the elevation frequency shift antenna illustrated schematically in Fig. l;

Figs. 7 and 8 are illustrative curves of the 'glide'path of the device of the present invention;

Figs/9 -and 10 show in partly-block form additional details of certain major components ofthe apparatus shown in Fig. 1;and

, Fig. 1l is a block diagram of the airborne installation portion of the system of the'present invention.

Referring now to Fig. 1, the portion of the system of the vpresent invention installed onthe ground cornprises azimuth and elevation frequency scan radars 10 `and 12, respectively,V which operate in conjunction with a frnaster control unit l18 which includes an 'azi'riinth-eleva' tion range indicator"19 for monitoring the position of the aircraft. The azimuth frequency scan radar( 10 includes an azimuth scan transmitter Z0,- an azimuth 'scan receiver 22, and an antenna schematically shown at 24. Similarly, the elevation frequency scan radar 12-includes A sociated antenna 34, on the other hand, may be located 1000 feetto one side of 'therun'way vand approximately 2000 feet ahead of the area where the aircraft should touch down. These distances are desirable, but it is recognized that not all airports are capable of providing this location conveniently. The above distances are-preferred to insure that 'thetouchdown area is outside of the -liiresnel `z'one 'of the-antenna. However, the eiit'n't of the Fresnelzone can be 'shortened by prefoeusirg'tlie antenna so`th`at theabo've distances can be considerably reduced. Ifdesirable, the 2000 Afeet displacement from the touchdown v'area 'may -be reduced -to 'the ei'rtent that tli'eelevation'antenn'a S21-may be-placed directly opposite the'beg'innin'g of the rnwa'y.

The elevation antenna '3`4 r'diates-a f'fan-shaped beam 'ne'Lhalf deg'i'eeinlelevation and -2-1 degrees in azir'nutll and in operation 'scans from nero degrees '-to seven r degrees-in elevation. vThis scan-is ac'icomplished in 'affequency bandwidth of approximately L'5 percent. iBoth the 'azimuth and -elevatinfrequency scan 'radars 10 and '12,respectively,have'thesaine'pulse repetition rate, frame an elevation scan transmitter 30, an elevation scan ref ceiver 32, and an antenna'34. The azimuth-and elevation radars 10, 12 are synchronized by means of=trans vmit triggers provided by the master control unit '1"8. `The azimuth and elevation video necessarylfor the 'azimuth-elevation range' indicator '19 of the rnast'reont'rol unitv 1s is, in improvised by the azimfhlandelvascan `i'ate 'andfi'n'iadditiom scan their respective frames simultaneously. Thus, -the scanning rate in y'degrees per unit'timesthree-tiniesas high 'for the azimuth fr'ada'r '10 "aSfOr-tlie elevation radar 1-2. 'Ihe'frequencyba'rids ofthe two radars are distinct soi that 4the two'anglar 'coordinates of any lposition inthe scanned approach volum'e are uniquely defined in teriris ofiv'two frequencies.

'Inorder to understand more clearly the operation ofthe azimuth and elevation-frequency scan antennas 24 "and 34, ra Vreview ofthecharacteristics -of a frequency scan antennaf-is herewith presented. 'Illustrated schematically in^Fig."4is a basicdevice 40 by which an antenna beam is'eased to`scan by elia'nge of operating frequency. The fdevicefl() comprises a linear array of slotlradiati'ng elements 42 disposedat periodic intervals along the narrow wall of a rectangular waveguide 44. The linear array of v:slotradiating elements `42 is fed by the waveguide 44 vvvvhichis arranged-in such a mnner'that the lengthof line between 'adjacent slots is Iarg'erthan the inter-element Espacing, a and a progressive shift of radians occurs between adjacent-slots. Under'these circumstances, the

-nglep ofv the `inain'fbeam Trelative Vto the broadside of th'e :'arr'ay "is:

1P=arc 'sin -2i (l) Ag'is the guide Wavelength at the vfrequency of operation v under consideration, and

4s is'evidentf'rm Equation 2,' the-beam angle versus "ffequeneysensitvty of ltlie array isvv proportional 't'o fs/a.

the' beam is'broadside,

. f antenna beam can be made to scan over a large angle with a small change in operating frequency; i.e., a large change in gl/ is produced for a small change in operating frequency. In addition to the Vscan frequency bandwidth, `several other factors must be considered in an array with a large ratio of s/a. Among the more important of these are the electromagnetic wave transit time effects, temperature sensitivity of the beam position and the waveguide vattenuation produced by the feed structure. For very shortpulses, the time required for the electromagnetic energy'to travel the length of the feed waveguide may be -comparable to the'length of the pulse, resulting in transient excitation of the antenna thereby producing broader beamwidths and higher side lobes than when the antenna is energized in a steady state condition.v It has been found, however, that when the transit time is less than 1S to 20 percent of the transmitter pulse length, no appreciable deterioration of the antenna pattern results.

The slot radiators 42 in the array are coupled to the waveguide 44 in a controlled manner, so that the beam shape in the plane of the array possesses the desired narrow beam and side-lobe characteristics. In the plane normal to the array, beam shaping is performed by conventional methods, that is, by means of horns, reliectors, o rf-a combination of the two methods.

An actual embodiment of the azimuth frequency scanning antenna 24 is shown in Figs. 5, 5a and 5b. The antenna 24 comprises a linear array 48 of slots in the broad face of a conventional X-band waveguide section 46. The slots of the array 48 are disposed periodically falong and parallel to the longitudinal axis of the waveguide section 46 on alternately opposite sides of the center line of the broad face. The linear array 48 of slots feeds .a primary horn 50, which in turn illuminates a parabolic cylinder 52. The parabolic cylinder 52 constitutes a reiicctor which is of the order of 14 feet in length and l foot in height to produce a one-way gain of approximately 38 decibels and resulting in the desired beam width of onehalf ,degree -in azimuth and seven degrees in elevation.

The linear array 48 of slots is cut in a straight portion of the waveguide section 46 with a ratio s/a=1, see Equation 2, thereby giving a minimum change in the azimuth angle of the beam for corresponding changes in the operating frequency. The minimum value of s/a is preferred to the transit time excitation of the array 48 so as to enable the use of a rvery short transmitter pulse length. In addition, the unity value of s/ a has other advantages in that the waveguide losses can be kept to about one-half decibel and the temperature sensitivity of the waveguide reduced to Where a temperature excursion of 120 centigrade results in a beam shift of only i 0.04 degree in azimuth. 'Ihe wave polarization of the antenna 24 is vertical. During operation, a frequency band of approximately 20 percent centered at 8500 megacycles per second is required to scan the beam over a 20 angle. The azimuth antenna 24 radiates a vertical fan-shaped beam which step-scans the runway in azimuth, each step being equal to the azimuth beam width, i.e., one-half degree. The antenna 24 is disposed so that the crossover point between two of its centermost beams defines a .localizer path 64 which is a vertical plane passing through the center of the runway as indicated in Fig. 3. Accordingly, the beam radiated by the azimuthantenna 24 is designated as the localizer beam.

An actual embodiment of the elevation frequency scanning antenna 34 is shown in Figs. 6 and 6a. The antenna `The horn 58 together with the detlecting vanes 60 illuminate a reflector 62 to produce a wide angle cosecant beam. It is to be noted that this particular antenna configuration is only one of several ways of producing the required --radiation pattern. Further, elevation-antenna 34 may be amasar prefocused by imposing a slight curvature to the array 54 in a plane normal to the direction of the runway, i.e., the array 54 together with 4its associated horns and reflectors is made concave towards the runway, thereby reducing the extent of the near zone of the radiation pattern. This prefocusing of the array S4 may permit the lateral displacement of the elevation antenna 34 from th runway to be substantially reduced.

The linear array 54 shown in Fig. 6 is approximately l2 feet in height and the reflector 62 live inches in width and tilted toward the take-olf end of the runway by an angle of the order of 1.5. With the aforementioned di'- mensions, the antenna 34 has a one-way gain of approximately 31.5 decibels, radiates a beam that has a beam width of one-half degree in elevation and 2.1 degrees in azimuth, and has an azimuth pattern shaped to cover the runway and approach volume of the airport as indicated by its beam pattern 63 in Fig. 3. The frequency sensitivity of the elevation antenna 34 is similar to that of the azimuth antenna 24 but the frequency scanning bandwidth employed is only l0 percent and is centered at 10,000 megacycles per second. Similar to the electromagnetic energy radiated from the array 48 of the azimuth antenna, the radiated energy from the linear array 54 of edge slots is also vertically polarized. Further', when a conventional X-band rectangular waveguide is employed for the feeds for both the azimuth antenna 24 and the vertical antenna 34, the temperature sensitivity of the antenna 34 is less than that of the azimuth antenna 24, due to the fact that the elevation antenna 34 operates at a slighly higher frequency.

The elevation antenna 34 of the elevation scan radar 12 is preferably located 1000 feet to one side of the runway and approximately 2000 feet ahead of the touch,` down area. These dimensions may -be reduced to satisfy airport space limitations by prefocusing the elevation au-V tenna 34 as previously described. In operation, the elevation antenna 34 radiates a horizontal fan-shaped beam which step-scans in elevation, each step being one-half the elevation beam width, i.e., one-quarter degree.

By way of explanation of the manner in which the glide path is produced consider first the radiation pattern of a linear array of isotropic radiators and let the array be long in terms of wavelengths. The radiation patterns produced by such an array are figures of revolution and are narrow in any plane containing the array. At broadside the radiation pattern or beam of such an array is shaped in the form of a disc. As the radiated beam of the array is scanned from broadside, the disc becomes the surface of a circular cone. The radiation pattern 63 (Fig. 3) of the elevation antenna 34 has the same characteristics as if it were a part of the radiation pattern of a linear array of isotropic radiators. elevation antenna 34 was placed in the center of the runway, the intersection between its radiation pattern 63, since it is a portion of a conical surface, and the localizer plane 64 would prescribe a glide -path which would be a straight line. By placing the antenna 34 to one side-of the runway, however, the intersection between its radiated beam and the localizer plane 64 becomes a hyperbola, as is shown in Fig. 7. For reasonable displacement of the antenna 34 lfrom the runway this hyperbola is too sharp to allow touchdown at its vertex because are-out (the region where the aircraft commences to assume a landing slope) would occur too close to the ground. In the system of the present invention, however, the elevation antenna 34 is tilted toward the take-off direction of the runway, thereby producing a very satisfactoryglide path and dare-out, as shown in Fig. 8. -v

By tilting the elevation antenna 34 in this mannen'a hyperbolic glide path is produced at the intersection with the localizer beam where one asymptote of the hyperbola constitutes the glide slope, the knee of the hyper-bola constitutes the flare-out, and the second asymptote becomes the landing slope. The actual amount of antenna Thus, if thev substantialdeviation from the glide path during are- 10 out. A landing slope of 0.3 corresponds to a vertical component of velocity ranging from l to 2. feet per second, for 60 to 240knots landing speed, respectively. Referring to Fig. 3, the -horizontal distance from areout to touchdown may-be approximately 3000 feet. A

decrease in the distance between the elevation antenna 34 and the runway will result in a modified but still usable landing glide path. In general, such a landing glide path will necessitate slightly higher vertical velocities, such as 3A to 3 feet per second, and higher accel- 20 erations at touchdown and flare-out respectively.

vReferring again to Fig. '1, the power for generating the'azimuth and elevationradiation patterns is provided by the azimuth and elevation scan transmitters and -30,- respectively. These two transmitters Iare-similar with the exception of their operating frequencies and bandwidth. 'By way of example, each transmitter may have a peak pulse power output of 50 kilowatts, a pulse width of 0.3 microsecond and a pulse repetition frequency 'of 6,500 pulses per second. Each of the transmitters 20 30 and comprises two sub-units; namely, microwave amplifiers 74, 76, 4and vexciters 78, 80, respectively. The generation of the precise -low level transmitter and reeeiver local oscillator signal frequencies is performed by -the exciters 78, 80 in Aresponse to the transmit trigger'signal from the master control unit 18. The low level transmitter signals from each of the exciters 78, 80 drives the microwave amplifiers 74, 76, respectively, the power output of each amplifier is the desired kilowatts. The receiver local oscillator signals from the exciters 78, 80

f' fare made available to the azimuth and elevation scan -receivers 22, 32, respectively. In addition, the azimuth Tand elevation receivers 22, 32 are coupled through du- -plexers to the azimuth and elevation antennas 2:'4, 34, respectively. Hence, in order that the receivers 22, 32 'be desensitized during intervals when microwave energy provided by the transmitters 20, 30 is lbeing radiated by -the antennas 24, 34, respectively, the receivers 22, 32 have impressed thereon the respective transmit vtrigger signals provided by the master control unit 18. Also the 'receivers 22, 32 provide the azimuth and elevation video necessary for the azimuth-elevation indicator 19 by means vof the connections therefrom over the leads 23, 33 to -tlie master control unit 18.

The exciters 718, 80 constitute the sources of the fre- -`quencies which determine the beam positions of the 'energy radiated from the vazimuth and elevation freq'u'ency scan antennas 24, 34, respectively. Theoutput 'frequencies 'of the transmitter signals from the exciters v78, A80 are preferably stable and have precise values,"so -'that a standardized 'receiving apparatus on the'aircraft frnay be made highly'selective during the final stages of the landing operation. It is to be noted that the position -of the pair of beams generated by the azimuth radar 10 which define the localizer plane 64 and the position of fthe p'air of beams generated by the elevation radar 12, ithe intersection of which with the localizer plane v64Y determines the landingpath'of the aircraft, are automati- "cally determined by the characteristics and attitude of the azimuth and elevation antennas 24, 34, respectively. Secondly, lthe exciters 78,'-80 provide local oscillator'sglnals for the duration of the listening period between 'transmitted pulses for the4 azimuth and elevation-receivers HL2, l32, respectively, which Ldiffer frequencywise 'i the signal to the microwave amplifiers 74, 76 by the 30 megacycles per second intermediate frequency of the receivers.

One example of an embodiment of exciter th-'at 'may be employed inthe elevation frequency sca-nning vradar 1 2 is described in 'o'rd'er to illustrate one manner in theabove requirements may be fulfilled. Atypicl'frequency band necessary to scan the seven-degree elevation 'sector covered by the'eleva'tion antenna :34 'is from v9,500 `to 10,470 -megacycle's "per second. If :the elevation Ibea'r'n, "for example, is Vmade to step-scan in 'onequarter degree increments throughout the above sector, it -'is necessary that'28 -dis'crete 'frequencies in'the above frequency band be generated. v d v Aiblock diagram 'of exciter `80 employed nthetranS- t'nitter'30 of the 'elevation frequency scanning radar12 is shown in `Fig.'9. As is generally known, thefrequ'en'cy of oscillation vof a'bakwardwave oscillator may beelec- 'tro'nieall'y tuned over 'an extremely broad band 'of 'frequencies, andfor a given oscillator, is determined by'the potential existing 'between -the helical slowwave structure and the'cathode-of'its 'associated electrongun.

'Referring to Fig. 9, thefex'citer'SO comprises a transmitter backward-'wave oscillator 81 -for generating the transmitter 'sgn'al'and a'receiver backward-wave oscillator 8,2 for generating ther'ee'eiver local oscillator signal'whieh operate'rncb'njunction with a low-voltage power supply 83, a stablepowersupply 84 and 'a step-voltage.'generator The transmitter `and receiver backward wave o'scillator 81 'andi8`2, respectively, 'may be of thetype ilescribed 'in a'w'pendiug application for patent, Serial No. l371,796, 'new abandoned, entitled Backward-WaveOscillator filed b'y Dean A. "Watkins on August `3, 1953, which linc: `rp"o''ra-tesfa lhelical slowawave structure. 'A backwardwave 'oscillatorof this type vhas a frequency versus helix voltage `vsensitivity of the orderof l megacycle' per second'p'e'r volt. 1In order to give a beam position'accuracy "of i005 degree for the `elevation scan radar, the'frequency must beheld within i-5 'mega'eycles perfse'cond -This frequencytolerance requires thepower sup'plyto be stable within :E5 volts, which is'no't difficult 'to'r'ealize The 'step-voltage generator 85 produces 'two voltages which remain constant during the intervals between trig- Lgersign'als'produced by anea'rly trigger generator 86 Va'nd which assume'28predetermined magnitudesin asuccession determined by the sequence in which it is desired lt'orradiate the signals of 28 discrete frequencies. -The 1early trigger generator `86 accepts the transmit trigger from the imastercontrol unit '18 and essentiallyperform's a time'delay 'toproduce an early trigger in the se'nse that it precedes, :in time, the succeedingtransmitter trigger. 'Ihis'early trigger isgenerated at the end of thereceivng'period which follows each'transmitted pulse.

The-two series f voltages produced by the step-voltage generator 85 "are impressed on the transmitter and receiverbackwar'd-'wave oscillators 81, 82 in a 'manerto produce the desired frequencies of oscillation. "-Durin'g -o'peratirin,`the"early`f:rigger effects a change in 'the-magnitude of thevoltages produced by the'step-voltagegen- 'erator'SSjgi'riortoI the instant 'each pulse is'transmited by Iavperiodlthat is =adequate to allow the system to recover 'from any transients'incident to the change in frequency. 'Also, 1an-automatic frequency control device `vv^S8 which is responsive to both the transmitter-signal generated by the oscillatorsl and-the receiver local oscillator signal `gerie'z'rate'd-by the'A'oscillatorSZ is connected to-the s'tep- @voltagefgenerator'e and functions in amanner to change the magnitude of the volt-age impressed on Ithe received backv'lardwave"oscillator `82 so that the frequencyf `tile"lo ':al 'oscillatorsignal differs from that of lthe trans- "mitter signal b'y-'the intermediate frequency of 'the'eleva- *tion receiver 32.

Theoutput 'signal from theitransr'nter" backward-Wave o's'cillator 81, -is `fed` to Ea travelingiwave' tube ampli'er 90. The rtraveliiigwvetube '-190 -s meaulated :by 1a narrow pulse 92 from the modulator in the microwave amplifier 76 so as to generate the drive signal of approximately 1 watt peak power required by the microwave amplifier 76.

'I'he exciter 78 employed in the azimuth transmitter 20 of the azimuth frequency scanning radar 10 may be similar to the exciter 80 of the elevation frequency scanning radar 12 described above, except that an additional number of transmitter and receiver signals is required to cover the larger angle of scan. A typical frequency band may, for example, range from 7,620 to 9,360 megacycles per second. It is evident that the additional frequencies may lbe generated by increasing the number of voltage steps generated by the step-voltage generator incorporated in the azimuth exciter 78.

The microwave amplifiers 74, 76 for the azimuth and elevation transmitters 20, 30, respectively, are similar except for their operating frequencies and bandwidths. Each amplifier has, for example, a 47 decibel gain, an output peak pulse power of 50 kilowatts and a pulse length of 0.3 microsecond. A schematic block diagram of the microwave amplifier 74 of the azimuth radar 10 is shown by way of example in Fig. 10. Referring to this gure, the microwave amplifier 74 includes a travelingwave tube buffer amplifier 96 which is driven by the Vone watt transmitter signal from the azimuth exciter 78.

'I'he buffer traveling-wave tube 96 has a gain of approximately 30 decibels and is similar to the traveling-wave tube amplifier employed in the exciter 78. During operation, the cathodes of both traveling-wave tubes are pulsed with the pulse 92 by means of a common modulator 98. The pulse 92 constitutes a 10 kilovolt negative pulse of 0.4 microsecond duration with rise and fall times of 0.3 microsecond, the rise of each pulse 92 being initiated by the transmit trigger from the master control unit 118.

A high power traveling-wave tube 100 capable of approximately 17 decibel gain provides the final amplifier stage. A tube of this type is disclosed in a copending application for patent, Serial No. 450,987, entitled High Power Microwave Tube, filed August 19, 1954 by Charles K. Birdsall. In the case of the traveling-wave tube 100,J

however, the transmit trigger from the master control unit '18 is first delayed by 0.35 microsecond by means of a delay line 102. The delayed transmit trigger pulse is then employed to trigger the output modulator 104 which, in turn,.energizes a 1:3 pulse transformer 106 to produce a 30 kilovolt, 0.3 microsecond pulse 108. This V30 kilovolt pulse 108 is impressed on the cathode of the traveling-wave tube 100 du-ring an interval that is coextensive with the duration of the pulsed transmitter signal from the buffer traveling-wave amplifier tube 96. The pulsed transmitter output signal from the tube 96 has an envelope as indicated in Fig. l that corresponds to the shape of the pulse 92 generated by the modulator 98, that is, a duration of 0.4 microsecond and a rise and fall time of 0.3 microsecond. Inasmuch as the 30 kilovolt pulse 108 has been delayed 0.35 microsecond, from the beginning of the rise of the pulse 92, the traveling-wave tube 100 amplification commencing 0.05 microsecond after the start of and ending 0.05 microsecond prior to the termination of the flat top or maximum value 0.4 microsecond portion of the pulse 92. From the above, it is apparent that the traveling-wave tube 100 has full microwave frequency excitation during the entire duration of the 0.3 microsecond, 30 kilovolt pulse 108. Thus, the output signal from the traveling-wave tube 100 illustrated in Fig. 10 and which is employed to energize the azimuth antenna 24, has a 0.3 microsecond duration, is of the same frequency as that generated in the exciter 78 and has a peak power of approximately 50 kilowatts.

The function of the airborne portion of the instrument approach and landing system of the present invention is to measure the frequency of the transmitted signals of the azimuth and elevation frequency scan radars and 12, which are incident on the aircraft. These measurements give the two basic angular coordinates of theI aircraft with respect to the runway, i.e., the azimuth and elevation angles. A block diagram of the portion of the apparatus installed on the aircraft is shown in Fig. 111. Referring to this figure, the airborne apparatus comprises an elevation channel 120 and a-n azimuth channel 122, which operate in conjunction with a common antenna 124 and a common glide and localizer path indicator The antenna 124 is a single flush-mounted. slot antenna capable of receiving the two ground transmitter signals. Since the antenna is mounted -a known distance above the wheels of the aircraft the angular 'posi'- tion measurements can be given in terms of the positions of the landing wheels above the runway. The elevation and azimuth channels 120, 122 are similar, except for the different frequency coverage.

The elevation channel comprises an elevation scan filter 128, the output of which is impressed upon both a coarse-discriminator 132 and a fine discriminator 134. A switch 136 enables the output from one of the discriminators 132, 134 to be selected and impressed upon a detector amplifier 138. The azimuth channel 122 is similar to the elevation channel 1120, comprising an muth scan filter 130 the output of which -is impressed upon both a coarse-discriminator 140 and a fine discriminator 142. A switch 144 enables the output of one of the discriminators 1-40, 142 to be impressed upon a detector amplifier 146.

The elevation and azimuth scan filters 128, 130 are of the band-pass type which are tuned to the frequency range employed by the corresponding radar system installed on the ground. The discriminators 132, 134 and 140, 142 provide video output voltages whose magnitude is a function of the frequency of the input signal. The' coarse discriminators 132, 140 may, for example, cover the entire frequency band of the corresponding ground based radar system whereas the fine discriminators may have `a frequency range, for example, of only 1 percent. The coarse and fine discriminators 132, -134 are, of course, centered so as to produce a null signal at a frequency midway between the frequencies of the two eleva', tion beams selected to produce the glide path. The coarse and fine discriminators 140, 142, on the other hand, are centered to produce a null signal at a frequency midway between the two azimuth beams selected to provide the localizer path. The output voltages from the discriminators together with the derivatives thereof, if desired, are used to provide the steering information for the glide and localizer path yindicator 126. The coarse discriminators 132, 140 are used in the initial approach phase where deviations of the aircraft from the landing course may be large. Automatic or manual means may be employed to throw the switches 136, 144 to the outputs of the fine discriminators 134, 142 when the ydelviations from the landing cou-rse become small, thus providing more accurate steering information for the pilot.

The detector amplifiers 138, '-146 of the elevation and azimuth channels 120, 122 convert video pulses received from the discriminators into corresponding direct-current voltages. The voltages thus generated by the detectoramplifiers 138, 146 are proportional to the deviation of the frequency of the received signal from the null'frequency of its associated discriminator and thus are indicative of the deviation of the aircraft from the desired landing course. The voltages developed by both the elevation'and azimuth channels 120, 122 which are indicative of the deviation from the desired landing course are presented in visual form to the pilot of the aircraft by means of the glide and localizer path indicator 126.

What is claimed is:

1. An instrument approach and landing system for aircraft comprising means disposed beyond one extremity of a runway across an extension of the longitudinal axis thereof for radiating a first plurality of thin vertical beams throughout a selected approach volume to the runway, 'the frequency of each of said vertical beams being a"r"stband of frequencies and being unique with respect 'to the azimuth thereof, whereby a first predetermined frequency 'withinsaid first band defines a localizer plane which contains said longitudinal axis and is perpendicularto the surface' of the runway; means including a verti- 'cal 'linear array of radiating elements disposed in the vicinity of the opposite extremity of the runway, tilted at 'a'predetermined angle towards said one extremity and `spaced 'from the longitudinal axis thereof for radiating a second plurality of thin tilted conically-.shaped beams throughout said approach volume, the frequency of each of said conically-shap'ed beams being within a second band of Lfrequencies -and being unique with Irespect to V'its angle of elevation, whereby the intersection between at least one conically-shaped "beam and said localizer 'plane defines a hyperbolic pathA whereby one asymptotic 'portion of said hyperbolic path provides a glide slope 'of an angle to the runway for the aircraft that is substantally equal to twice said predetermined angle, where- Tbythe knee portion of said hyperbolic path provides a flare-'out region for said aircraft, and whereby the remaining asymptotic portion of said hyperbolic path pro- Yvides a landing slope to the runway for the aircraft, said landing slope being substantially equal to one-fifth said predetermined angle thereby enabling an aircraft pos- 'se'ssing means for receiving said first and second predeter-mined frequencies to be directed along said path.

2. An instrument approach -and vlanding system for 4aircraft comprising means disposed'beyond one extremity of a runway across an extension of the longitudinalaxis 'thereof for radiating -a first plurality of thin vertical `beamsthroughout aselected approach volumeto the run- Way, the frequency of each of said verticalbeams'being within a'first band of frequencies and being unique with respect to the azimuth thereof, whereby a first predetermined. frequency within said first band defines a local- .'izerplane which contains said longitudinal axis and is perpendicular to the surface of the runway; means inluding a vertical linear array of radiating elements disposed in the vicinity of the opposite vextremity of the runway, spaced from lthe longitudinal axis thereof and curved inwards toward and in aplane normal to said .runway and spaced from the longitudinal thereof for radiating a second plurality of thin prefocused conicallyshaped beams throughout said approach volume, the frequency of each of said conicaIly-shaped beams being with- ;in a second band of'frequencies and'being unique with respect'to its angle of elevation whereby the intersec- 4'tion between Ithe cross-over of -two selected adjacent conically-'shaped beams corresponding to a second predetermined frequency within said second band, and said Alocalizer plane defines ahyperbolc path thereby to enable an aircraft possessing means for producing null -indications at said first and second predetermined frequencies to be directed along said path.

3.The instrument approach and landing system for aircraft as defined in claim l wherein saidpredetermined angle is substantially equal to 1.5

4. An instrument approach and'landing -systemvfor aircraftfcomprising -an azimuth frequency-scanning antenna disposed beyond one extremity ofa runway across an -extension-of-the longitudinal axisthereof, the radiation ,patternof said azimuth antenna being a thin vertical beam yhaving an azimuth direction dependent uponthe .frequency of the microwave energy with which -said =azimuth antenna is energized; means for -periodically energizing said a'zimuthantenna with pulsed-microwave signals of successively different frequencies-within a first frequency'band to step-scan said narrow vertical beam across Ia selected 'approach volumeI to the runwaygwherel"-by'a first predetermined frequency withinsaid'first band fi'nes a"localizer plane-which contains saidlongitudinal axis and Avis normal to the surface of said runway; an 'elevation 'frequency-scanning antenna disposed 'in the vicinity vof the `opposite extremity of the runway and spaced from'thelongitudinal axis thereof, the portion of the .radiation pattern of said elevation antenna Within 'said approach volume being a tilted fiat conically-shaped vbeam with a vertex coinciding with the position of said elevation antenna, ,and the angle of elevation of said conically-shaped 'beam 'being dependent upon the frequency of 'the microwave energy with which said eleva- 'tion antenna -islenergized; means 'for periodically energizing said elevation antenna with pulsed microwave signals of successively different frequencies Within a .second frequency band to step-scan said tilted conicallyshaped beam in elevation, whereby the intersection'between the cross-over of two selected adjacent fiat beams, corresponding' to a second predetermined frequency within said second band, and Vsaid localizer plane defines a tilted hyperbolic descent path; and means disposed on lboard anaircraft and tuned to said first and second predetermined 'frequencies for producing signals to enable the aircraft to be directed along said descent path.

5. The instrument Vapproach and Alanding system for aircraft as defined'in claim 4 which additionally includes means coupled to said azimuth antenna for receiving echo signals in response to the vertical beams radiated therefrom and reflected from the aircraft, means coupled to said elevation'antenna for receiving echo signals lin-response to the [dat conically-shaped beams radiated therefrom and-reliected-from the aircraft, andmeans responsive to saidecho .signals 4forproviding a visualpresenta- :tion of Vthepositioncf the aircraft with respect tothe runway.

6. The instrument-approach and landing system 'for aircraft as defined in claim 4 wherein said azimuth frequency-scanning antenna includes a straight section of rectangular waveguide disposed in a horizontal position wherein one of the broadsides of said waveguide defines a series 'of periodically spaced slots, thereby toprovide a linear array of radiating elements, said .slots being disposed .parallel to-the'longitudinal axis of said waveguide and-on alternately opposite .sides of the center line of said one'broad side.

7. The instrument approach and landing system for aircraft as defined in cl-aim 4 wherein said elevation'frequency-scanning-an-tenna includes a rectangular waveguide section mounted in a substantially vertical position wherein one of the narrow sides of said waveguide section definesaseries of periodically spaced slots, thereby to providea lineararray of radiating elements.

8. 'In an instrumentiapproach and landing system for aircraft having apparatus disposed beyond one extremity of a runway along an extension of the'longitudinal axis thereof for radiating va first plurality of thin vertical beams throughouta selected approach volume to 'the runway, the.frequencyiof each of said vertical vbeams being within ai first band of'frequencies and unique with .respectto the :azimuth thereof, and apparatus disposed beyond'theopposite extremity of the runway and spaced 'from thelongitudinal axis thereof for radiating a second plurality of 'thin fiat beams throughout said approach volume,"the frequency of`each of'saidflat beams being within a secondbandof frequencies and unique withv respect to'its -angle of elevation, a system disposed on board said aircraft for determining its angular position with respect'torsaid runway comprising: an antenna vdis- 'posed 'on said aircraft `at 'arpredetermined distance above inE -thetfrequency ofsadreceived velectromagnetic Lenergy 13 within said first and second bands of frequencies from rst and second preselected frequencies Within said rst and second bands, respectively.

References Cited in the le of this patent 14 Brockstedt July 5, 1932 Herzog June 25, 1940 Alford Sept. 8, 1942 Green Dec. 26, 1944 ONeil June 12, 1945. Wilkie Sept. 29, 1953 UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No2. 2952845 september 13, 1960 Nicholas A. Begovich et al.

It is hereby certified that error appears in the printed 4specification i of the above num bered patent requiring correction and that the said Letters Patent should read as corrected below.

l column 8, line 5l*7 for "transmitter" read transmlt u line 56, for "manor" read M manner line 68, for "received read receiver --t Signed and sealed this llth day of April 1961.

(SEAL) Attest: v

ERNEST W- SWDER ARTHUR w. CROCKER Attesting Oicer Acting Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent, 2952845 september le1 1960 Nicholas A. Blegovieh en al@ of 'the above numbered patent requiri Paten-'b should read as corrected belo Column4. line 7Y after "onemhalf" insert degree =-=f column 8g line 5l for transmitter read en' transmit ma; line 56, for "maner" read @n manner =eg line 68 for .received" read receiver 1 Signed and sealed this llth day of April 1961.

(SEAL) Attest:

SWIDER Y ERNEST W ARTHUR vv. CRoCKER Attesting Ocer Acting Commissioner of Patents 

